Anyone has an idea what could be causing this? The files exist and seem to have the right permissions, but there really seems to be something wrong with the /dev/dsp symlink, else osstest couldn't work, right?

This is odd... Lets try this:Edit /usr/lib/oss/conf/osscore.conf and add the line "vmix_no_autoattach=1". Then restart OSS (soundoff && soundon). First, see if ossplay/etc. work now. If they do, try attaching vmix using vmixctl command. Then try ossplay again.

Sorry for taking so long to reply.. You better provide the second argument to vmixctl (an input node - e.g. pcmin0), otherwise programs opening the device with O_RDWR would fail. You can also try using -M switch with vmixctl.